1. Field of the Invention
The present invention relates to a photodetector module for converting an input optical signal into an electrical signal and outputting the electrical signal.
2. Description of the Related Art
On the receiving side of a general optical communication system, an optical signal transmitted through an optical waveguide is opto-electrically converted into an electrical signal by a photodetector such as a photodiode, and information is reproduced according to the electrical signal obtained. The photodetector is a device indispensable to an optical communication system and an information processing system for processing massive amounts of information. To put such systems into widespread use in the general public, it is necessary to develop a technique capable of providing an optical waveguide for transmitting an optical signal and a photodetector for converting the optical signal into an electrical signal with a high efficiency at a low cost.
FIG. 1 shows a sectional view of a photodetector module 2 in the prior art. The photodetector module 2 includes a photodetector 6 mounted on a support substrate 4. The photodetector 6 is formed by sequentially laminating an n-type InP buffer layer 10, an InGaAs light absorbing layer 12, and an n.sup.- InP layer 14 on a substrate 8 of n-type InP by MOCVD, for example. Two p-type regions 16 and 18 are formed in the n.sup.- InP layer 14 by thermal diffusion of zinc, for example. Two p electrodes 20 and 22 are formed on the surfaces of the p-type regions 16 and 18, respectively.
The p-type region 16, the InGaAs light absorbing layer 12, and the n-type InP buffer layer 10 constitute a pin photodiode (pin-PD) 24. Similarly, the p-type region 18, the InGaAs light absorbing layer 12, and the n-type InP buffer layer 10 constitute a pin-PD 26. The n-type buffer layer 10 serves as a common n electrode. Thus, the two pin photodiodes 24 and 26 are arranged in parallel with the common n electrode on the whole of the photodetector. A bias voltage is applied between the p electrodes 20 and 22 by a bias power supply 28 to apply a reverse bias to the pin-PD 24 on which light is incident.
Inclined surfaces 34 and 36 are formed at a lower end portion of the substrate 8 on the opposite sides. An optical waveguide 38 is mounted on the support substrate 4, and a light beam 40 emerged from an end face 38a of the optical waveguide 38 is refracted by the inclined surface 34 and transmitted through the substrate 8 of the photodetector 6 to enter the pin photodiode 24. A portion of the InGaAs light absorbing layer 12 immediately below the p-type region 16 functions as a photodetecting portion 30. When light enters the photodetecting portion 30, electron-hole pairs are generated. These electrons and holes are moved by an electric field due to the above-mentioned bias voltage to bring about a flow of electric current having an intensity proportional to the intensity of the incident light through a resistor 32. The current is taken out as a voltage signal across the resistor 32.
In the conventional photodetector module 2 shown in FIG. 1, the light beam 40 is obliquely incident on the photodetecting portion 30. Accordingly, if the thickness of the substrate 8 deviates from a given thickness, the position of the light beam on the photodetecting portion 30 is largely deviated to cause a reduction in optical coupling efficiency. To prevent the reduction in optical coupling efficiency, the area of the photodetecting portion 30 must be made large. However, the enlargement of the photodetecting portion 30 causes an increase in capacitance of the photodetector, which results in deterioration of response characteristics. Further, since the incident angle at the interface between the light absorbing layer 12 and the buffer layer 10 is large, the reflectivity at the interface differs according to polarization, causing polarization dependence of the output beam from the optical waveguide 38.